Two documents of the state of the art use the just described scheme and, in addition, aim at separating the effects of pure rotation from spurious effects of translation and eccentricity. A fully optical compensation solution was disclosed by Canon (EP 0 589 477). A number of micro-optical elements arranged above a standard rotating encoder disk forward the diffraction orders produced by a first diffraction event on the encoder disk ring to another diametrically opposed part of the encoder disk ring where they experience a second diffraction event, the grating diffraction order used in the first and second diffraction events having the same direction relative to the direction of rotation. This implies that a translation displacement tangential to the disk at the location of the two diffraction events does not give any signal since the twice diffracted beam experiences two phase-shifts of opposite sign. The proposed solution is however very bulky and the mounting of all spare optical elements results in very high fabrication and mounting costs.
A more monolithic solution was disclosed by Heidenhain (Winkelmesseinrichtung, DE 38 36 703) which integrates the light circulation scheme bringing the light beams from one diffraction event to a second diffraction event diametrically opposed on the encoder disk in a grating coupled planar waveguide defined on a planar substrate placed above the encoder disk. The proposed solution represents a progress relative to the prior art, but grating coupling in a waveguide is very sensitive to the parallelism between the disk and light circulation waveguide, and is highly sensitive to the wavelength of the laser source used which therefore must be wavelength stabilized. It is also sensitive to the spacing between the disk and the waveguide substrate and requires careful mounting, therefore mounting costs.
Both the described solutions of the state of the art suffer from the fact that the optical hardware for the light beam circulation scheme from one position of the radial grating disk to the other is somehow fixed to the non-rotating body of the rotation encoder, and that its position relative to the encoded grating track on the rotating disk varies with the occurrence of spurious translation effects and during rotation if there is eccentricity between the housing and the rotation axis and/or between the rotation axis and the radial grating track. This implies that translation effects are not exactly compensated, and that the interfered optical wave front picked up by a detector at the output of the double diffraction scheme exhibits fringes which may lead to contrast fading.
For the interference contrast to be close to one (1.0), the relative positioning of the two radial gratings must be precise and stable during relative rotation. The smaller the encoder disk, the more critical the relative centering conditions. This implies in particular that the length of the radial grating lines must be short if an acceptable contrast is to be preserved upon relative rotation in the presence of eccentricity due to imperfect alignment and mechanical shocks or vibrations; this in turn implies that the fraction of optical power impinging onto the encoder disk which experiences diffraction and interference decreases with a decrease of the encoder disk diameter. As the disk diameter reduces, as in micromotors for instance, the requirement of an optimal use of the available incident optical power and the requirement of high interference contrast become increasingly contradictory in sensors of the state of the art where the grating is formed at a planar surface of an encoder disk. Furthermore, in sensors of the state of the art which aim at compensating the spurious translation or eccentricity effects optically, as disclosed in the above mentioned documents, the translation effect which one intends to eliminate in the measurement does degrade the interference contrast, therefore does compromise the very first objective which is the measurement of rotation.
A rotation sensor using a cylindrical grating on a rotating rod is known from the document DE 196 37 625 A1 (entitled “Motor oder Generator mit zwei relativ zueinander beweglich gelagerten Teilen and einer Vorrichtung zum interferometrischen Messen eines Weges oder Drehwinkels”). This document discloses an external optical read head with two planar wave front shaping gratings for the measurement of the rotation of a reflection grating or hologram written directly on the cylindrical wall of a rotating rod or shaft. The rotation sensor has a light source generated a highly diverging beam which is then separated into two converging beams by the two planar wave front shaping gratings. These two converging beams are then incident on a same spot of the cylindrical grating where they experience a diffraction in a same output direction before reaching a light detector arranged between said two planar gratings. Chi-Tang Hsieh and Chih-Kung Lee, in “Cylindrical-type nanometer-resolution laser diffractive optical encoder”, Applied Optics, Vol. 38, 1999, pp. 4743-4750, disclose an external complex read head with two sets of modified telescopes and wave front shaping means formed by several optical elements to compensate for radial-grating-induced wave front aberration. The only grating in this system is a cylindrical grating arranged at the lateral surface of a rotating disk. The several optical elements are precisely arranged so that two diffraction events occur successively on the single cylindrical grating. Both designs adapt the Cartesian geometry of a standard translation sensor read head to the symmetry of a rotating circularly symmetrical body which results in a complex optical system and cumbersome optical hardware. Further, both rotation sensors are very sensitive to any displacement of their optical elements, in particular to spurious translations or small rotations of these optical elements and to eccentricity of the rotating disk or rod.
U.S. Pat. No. 5,696,374 discloses an optical encoder for a linear position measuring system with the features of the preamble of claim 1.